We investigate the feasibility of correlating an optical cavity field and a vibrational phonon mode. A laser pumped quantum dot fixed on a nanomechanical resonator beam interacts as a whole with the optical resonator mode. When the quantum dot variables are faster than the optical and phonon ones, we obtain a final master equation describing the involved modes only. Increasing the temperature, which directly affects the vibrational degrees of freedom, one can as well influence the cavity photon intensity, i.e., the optical and phonon modes are correlated. Furthermore, the corresponding Cauchy-Schwarz inequality is violated demonstrating the quantum nature of those correlations.

We demonstrate experimentally that bright and nearly collimated radiation can be efficiently generated in air pumped by an intense femtosecond Bessel beam. We show that this nonlinear conversion process is driven by the rotational Raman response of air molecules. Under optimum conditions, the conversion efficiency from the Bessel pump into the on-axis propagating beam exceeds 15% and is limited by the onset of intensity clamping and plasma refraction on the beam axis. Our experimental findings are in excellent agreement with numerical simulations based on the standard model for the ultrafast nonlinear response of air.

We describe phase-dependent wavelength scaling of high-order-harmonic generation efficiency driven by ultrashort laser fields in the midinfrared. We employ both numerical solution of the time-dependent Schrödinger equation and the strong field approximation to analyze the fine-scale oscillations in the harmonic yield in the context of channel-closing effects. We show, by varying the carrier-envelope phase, that the amplitude of these oscillations depends strongly on the number of returning electron trajectories. Furthermore, the peak positions of the oscillations vary significantly as a function of the carrier-envelope phase. Owing to its practical applications, we also study the wavelength dependence of harmonic yield in the “single-cycle” limit, and observe a smooth variation in the wavelength scaling originating from the vanishing fine-scale oscillations.

We found a cheating strategy for the quantum cheat-sensitive bit commitment protocol proposed in Phys. Rev. A 84, 022308 (2011). It enables the receiver of the commitment to cheat with a probability not less than $1/2$, no matter what values are chosen for the security parameters in the protocol. More interestingly, the probability for successful cheating will rise as the number $n$ of the particles used in the protocol increases and will be arbitrarily close to $100\%$ in the limit $n\to \infty$.

In an article in 2007, Mishra et al. [Phys. Rev. A 76, 013604 (2007)] investigated the two-component Bose-Hubbard model using the numerical density-matrix renormalization-group procedure. In the regime of interspecies repulsion $U^{ab}$ larger than the intraspecies repulsion $U$, they found a transition from a uniform miscible phase to phase separation occurring at a finite value of $U$, e.g., at around $U=1.3$ for $\Delta =U^{ab}/U=1.05$ and ${\rho }_a={\rho }_b=1/2$. In this Comment, we show that this result is not correct, and in fact, the two-component Bose-Hubbard model is unstable to phase separation for any $U^{ab}>U>0$.

We consider high-order above-threshold ionization of polyatomic molecules by a strong laser field. An improved molecular strong-field approximation which takes into account the electron rescattering off the molecular centers is developed. The presented theory is applied to calculate the photoelectron energy and angular distributions for the ozone molecule. The obtained spectra exhibit pronounced minima, and this is explained as a three-point destructive interference of the rescattered electron wave packets.

We investigate coherent single-microwave-photon transport in the coupled superconducting transmission lines coupled to superconducting quantum interference device based charge qubits. The Fano resonance can be observed in the scattering spectra in both the linear and nonlinear regimes of the dispersion relation of the microwave photon. We further find that the degree of the two-qubit entanglement can vary from unity to zero when the Fano resonance occurs. There exists a correspondence between the entanglement and the Fano resonance.

We demonstrate that what is called a total photoionization cross section in the work by T. W. Gorczyca, M. F. Hasoglu, and S. T. Manson [Phys. Rev. A 86, 033204 (2012)] is in fact a partial photoionization cross section. These quantities differ impressively even in isolated atoms. This demonstrates the prominent role of inelastic collisions of a photoelectron from an intermediate or inner shell not mentioned in their paper. We discuss briefly the correspondence between the experimental data, theoretical predictions, and $R$-matrix results from Phys. Rev. A 86, 033204 (2012). We call attention to the danger in using parameters of C${}_{60}$ potential selected by fitting the same experimental data that one wants to describe. We show that our criticism is applicable to the theoretical part of the most recent publication on photoionization of $\mathrm{Xe}@{\mathrm{C}}_{60}^{+}$ [R. A. Phaneuf et al., Phys. Rev. A 88, 053402 (2013)].

Different operators have been suggested in the literature to describe the electron's spin degree of freedom within the relativistic Dirac theory. We compare concrete predictions of the various proposed relativistic spin operators in different physical situations. In particular, we investigate the so-called Pauli, Foldy-Wouthuysen, Czachor, Frenkel, Chakrabarti, Pryce, and Fradkin-Good spin operators. We demonstrate that when a quantum system interacts with electromagnetic potentials the various spin operators predict different expectation values. This is explicitly illustrated for the scattering dynamics at a potential step and in a standing laser field and also for energy eigenstates of hydrogenic ions. Therefore, one may distinguish between the proposed relativistic spin operators experimentally.

The energy of the ${\mathrm{He}}^{-}$ ion in a magnetic field is calculated using the stochastic variational method with a deformed correlated Gaussian basis. The energy levels and the stability domains are calculated and the accuracy of the approach is shown by comparison to previous calculations. The structure of these states is studied by calculating the electron-electron and electron-nucleus distances. In the weak field case, these systems have a pronounced inert He core plus a weakly bound electronlike structure. Stronger fields lead more compact systems unless the stability is lost due to level crossings.

An ambiguity of the separation of cascades from “pure” two-photon decay is confirmed with accurate numerical calculations in a gauge-invariant way. A direct evaluation of the two-photon decay width of excited states in H-like ions via the imaginary part of two-loop self-energy is presented. We demonstrate that there is a fundamental difference between the level width and the transition probability in the presence of cascades. The two-photon widths are shown to be different from the two-photon decay rates for the transitions including cascades.

The integration of lab-on-a-chip technologies with single-molecule detection techniques may enable new applications in analytical chemistry, biotechnology, and medicine. We describe a method based on the reciprocity theorem of electromagnetic theory to determine and optimize the detection efficiency of photons emitted by single quantum emitters through truncated dielectric waveguides of arbitrary shape positioned in their proximity. We demonstrate experimentally that detection of single quantum emitters via such waveguides is possible, confirming the predicted behavior of the detection efficiency. Our findings blaze the trail towards efficient lensless single-emitter detection compatible with large-scale optofluidic integration.

We investigate the effect of near threshold resonances in reactive scattering at low energy. We find a general type of anomalous behavior of the cross sections, and illustrate it with a real system (${\text{H}}_2+\text{Cl}$). For inelastic processes, the anomalous energy dependence of the total cross section is given by $\sigma \sim k^{-3}$ (with $k\propto \sqrt{\varepsilon }$). The standard threshold behavior given by Wigner's law ($\sigma \sim k^{-1}$) is eventually recovered at vanishing energies, but its validity is now limited to a much narrower range of energies. The anomalous behavior leads to reaction rate coefficients behaving as $K\sim 1/T$ instead of the expected constant rate of the Wigner regime. We also provide analytical expressions for $s$-wave cross sections, and discuss the implication in ultracold physics and chemistry.

Dephasing decoherence induced by interaction of one qubit with a quantum bath can be simulated classically by random unitary evolution without the need for a bath and this random unitary evolution is equivalent to the quantum case. For a general dephasing model and a single-qubit system, we explicitly construct the noise functional and completely specify the random unitary evolution. To demonstrate the technique, we applied our results to three paradigmatic models: spin-boson, central spin, and quantum impurity. For multiple qubits, we identify a class of generalized quantum dephasing models that can be simulated classically. Finally we show that depolarizing quantum models can be simulated classically for all dimensionalities of the principal system.

The minimum-Kullback-entropy principle (mKE) is a useful tool to estimate quantum states and operations from incomplete data and prior information. In general, the solution of an mKE problem is analytically challenging and an approximate solution has been proposed and employed in different contexts. Recently, the form and a way to compute the exact solution for finite dimensional systems has been found, and a question naturally arises on whether the approximate solution could be an effective substitute for the exact solution, and in which regimes this substitution can be performed. Here, we provide a systematic comparison between the exact and the approximate mKE solutions for a qubit system when average data from a single observable are available. We address both mKE estimation of states and weak Hamiltonians, and compare the two solutions in terms of state fidelity and operator distance. We find that the approximate solution is generally close to the exact one unless the initial state is near an eigenstate of the measured observable. Our results provide a rigorous justification for the use of the approximate solution whenever the above condition does not occur, and extend its range of application beyond those situations satisfying the assumptions used for its derivation.

We consider a potential well whose shape changes with time, so that a bound state is brought closer to the edge of the continuum. We then ask the following: how slow should the evolution be for the adiabatic approximation to hold? We answer the question by considering, in a Sturmian representation, linear evolution of a $\delta$-function potential well. The general applicability and accuracy of the obtained adiabatic criterion is discussed.

We introduce a variation of the discrete-time quantum walk, the nonreversal quantum walk, which does not step back onto a position that it has just occupied. This allows us to simulate a dimer and we achieve it by introducing a different type of coin operator. The nonrepeating walk, which never moves in the same direction in consecutive time steps, arises by a permutation of this coin operator. We describe the basic properties of both walks and prove that the even-order joint moments of the nonrepeating walker are independent of the initial condition, being determined by five parameters derived from the coin instead. Numerical evidence suggests that the same is the case for the nonreversal walk. This contrasts strongly with previously studied coins, such as the Grover operator, where the initial condition can be used to control the standard deviation of the walker.